The present invention generally relates to alarm systems, and more specifically to a voltage enhancement circuit for central station or police monitored alarm systems.
Frequently, an alarm system, whether residential or commercial, is connected to a police or other remote monitoring location by means of telephone lines. This often represents the least expensive means for making such a connection and eliminates the need for laying down new lines between the two locations which are to be in communication with each other. However, because the telephone lines invariably go through one or more central switching stations before connection is made to the desired location, this normally substantially increases the effective distance or length between the connected locations. Accordingly, the total length of the telephone cables which are required between each residential location, for example, and a police or monitoring station will vary in each case. Sometimes, because of the peculiar arrangement in which the telephone lines are initially set down, it may even be necessary to go to two or more central telephone stations before connection is made to the desired location. For these reasons, the total length of telephone cable between the protected area and the monitoring area cannot be predicted in advance until these protected and monitoring locations are selected.
Due to the substantial variations in telephone line length and therefore telephone line internal resistance between two interconnected locations, it has been a problem in the prior art insofar as compensating for the additional lengths of telephone cable which, as suggested, may vary from location to location. It is not uncommon, for example, for the telephone line resistance to vary over wide ranges, such as between 3K ohms and 10K ohms. Several prior art approaches for compensating for the voltage drops across telephone line resistance will now be briefly described.
The "police station" circuit is basically simple and includes a power supply which converts alternating house power to a DC voltage. In the event that house power is lost, conventional means may be provided for actuating an auxilliary power switch which places a stand-by battery into the circuit in place of the power supply to automatically take over the function of the power supply. A current limiting resistor is advantageously placed in series with the source of DC voltage at the monitored location for limiting the current drawn from the same in the event of a short circuit to thereby prevent damage to the power supply.
A major problem which has been encountered in the use of the monitoring systems under discussion has resulted from the frequently high internal telephone line resistance which has resulted in substantial voltage drops across the telephone lines leaving little voltage to be applied across the police station indicating mechanism, such as meter terminals. Application of very low voltages below the minimum desired detectable level, has resulted in false alarm indications which require investigation. Clearly, such false alarms increase the inefficiency of the system and also the attendant expense in operating the same.
The power supply and stand-by battery have nominal voltage outputs. Thus, when the telephone line resistance is abnormally high and a substantial portion of this nominal voltage is dropped across the telephone line, the voltage of the output of the lines may drop below the minimum desired detectable level. For the above reasons, there have been made several attempts for compensating for the drops of voltage across the internal line resistance of the interconnecting lines. Typically, the method used has consisted of utilizing additional batteries or power supplies, connected in series with the basic power supply, which are added after the system has been installed on location and the total telephone line length and internal resistance are determined. The additional sources of voltage are intended to compensate for the voltage drops. However, the use of batteries, for example, has presented a problem since these must constantly be replaced. Additionally, when supplemental batteries of this type are used to compensate for the telephone line drops and these batteries lose their charge, they represent very high series resistance which, in this condition, only aggravate the problem of high telephone line resistance. Instead of enhancing the line voltage when the supplemental batteries discharge, they actually still further decrease the voltage which is available at the police or monitoring station.
Another prior art approach at compensating for the voltage drops across the telephone lines has been to utilize a second power supply which is placed in series with the alarm voltage supply at the monitored location. However, this substantially increases the cost of the alarm and, since it is dependent on AC house current, is subject to power failure and, therefore, also requires stand-by batteries.
With the prior art methods of compensating for the telephone line voltage drop, the solutions to the problem are only achieved by the introduction of new problems.